1. Field of the Invention
The present invention relates in general to fuel assemblies for nuclear reactors, and relates in particular to a fuel assembly for preventing abrasion of fuel rods caused by vibrations in the fuel assembly.
Background of the Invention
A known example of a fuel assembly for a pressurized light water reactor is shown in FIG. 2.
The fuel assembly shown in this drawing is provided with an upper nozzle 1, and a lower nozzle 2 which is oppositely spaced apart from the upper nozzle 1. A plurality of control rod guide pipes 3 are fixedly disposed between the upper nozzle 1 and the lower nozzle 2, and a plurality of support grids 4 are attached to the control rod guide pipes 3 in the middle section of the fuel assembly with a certain spacing, and a plurality of fuel rods 6 are inserted through and supported by the support grids 4.
As shown in FIG. 3, each of the support grids 4 comprises a plurality of straps 7 crossing at right angles to each other to produce a grid network and a plurality of square-shaped grid cells 5 therein. A plurality of dimples 9 and springs 10 are provided on the opposing walls surrounding each of the grid cells 5 to elastically hold the fuel rods 6 which are inserted through the grid cells 5.
The dimples 9 and the springs 10 are disposed in the grid cells 5 in the pattern shown in FIG. 3. Dividing the support gird 4 in a plan view into four quadrants S1, S2, S3 and S4 in a counter-clockwise designation about an origin O, equivalent to the center of the support grid 4. In the first quadrant S1, of the four walls defining each square-shaped grid cell 5, the dimples 9 are formed on the left and bottom walls while the springs 10 are formed on the right and top walls. In the second quadrant S2, the dimples 9 and the springs 10 are formed symmetrically to those in the first quadrant S1 with respect to the Y-axis. In the third quadrant S3, the dimples 9 and the springs 10 are formed symmetrically to those in the first quadrant S1 with respect to the origin O. In the fourth quadrant S4, the dimples 9 and the springs 10 are formed symmetrically to those in the first quadrant S1 with respect to the X-axis.
The fuel assembly comprising such support grids 4 is provided with a plurality of mixing vanes 15 formed at the intersections of the straps 7 (in the Figure, only a part of mixing vanes 15 are illustrated), for the purpose of stirring the cooling water for removing the heat generated by the fuel rods efficiently.
It is thought that, in a plan view of the grid, the turbulent flows caused by the presence of these mixing vanes are responsible for generating systematic vibrations of the fuel assembly in the diagonal direction shown by an arrow in FIG. 3.
When the fuel assembly vibrates in the diagonal direction, the following problem arises.
Specifically, considering the fuel rods 6 being held in the grid cells 5 which lie in the second and fourth quadrants, S2, S4, it can be seen that the fuel rods 6 are being supported by the dimples 9 in one direction of vibration, and by the springs 10 on the return direction of vibration. Because the dimples 9 have a higher resistance to deformation than that of the springs 10, there is a difference in the support being provided to the fuel rods 6 in the forward and reverse cycles of vibrations.
When the resistance to deformation of the supporting members differs in the forward and reverse cycles of vibration, if vibrations should be generated in the fuel assembly, the support grids 4 also vibrates, and the fuel rods 6 being held in the grid cells 5 disposed in the second and the fourth quadrants will exhibit a phenomenon of bouncing between the pairs of dimples 9 and the opposing springs 10. The fuel rods 6 under such a condition will experience abrasion caused by the fuel rods 6 rubbing against the dimples 9 and the springs 10.